This invention is directed to noise reduction, and more particularly, to an apparatus and method for performing noise reduction for a signal received at a microphone array.
A noise reduction apparatus is typically used in conjunction with hands-free mobile terminals (for example, cellular telephones) and speaker phones, or with speech recognition systems, to reduce noise received at a microphone array of the noise reduction apparatus.
The general structure of different array processing algorithms for noise reduction apparatuses utilizing microphone arrays in conjunction with signal processing can be expressed in the frequency domain as             U      out        ⁢          (      ω      )        =            ∑              i        =        1            N        ⁢          xe2x80x83        ⁢                  U        ⁢                  (                      ω            ,                          r              i                                )                    ·                        H          *                ⁢                  (                      ω            ,                          r              i                                )                    
where Uout(xcfx89) and U(xcfx89, r1) are respectively the Fourier transform of the microphone output and the field u(t, ri) observed at the i-th microphone elements with the spatial coordinates ri, H(xcfx89, r1) is the frequency response of the filter at the i-th element of the microphone array, and N is the number of microphone array elements.
The determination of the functions H(xcfx89, r1) is the major area of concern in array processing. In conventional array processing, the optimization criteria used for the determination of the functions H(xcfx89, ri) are based on an assumption that the signal field in a limited space, for example an automobile cabin, has a coherent structure. This assumption leads to the following conventional algorithm for the determination of the weighting functions H(xcfx89, r1):             H      ⁢              (                  ω          ,                      r            i                          )              ≡                  H        0            ⁢              (                  ω          ,                      r            i                          )              =            ∑              p        =        1            N        ⁢          xe2x80x83        ⁢                            K          N                      -            1                          ⁢                  (                                    ω              ;                              r                i                                      ,                          r              p                                )                    ⁢              G        ⁢                  (                                    ω              ;                              r                p                                      ,                          r              0                                )                    
where KNxe2x88x921(xcfx89, r1, rp) denotes the elements of the matrix KNxe2x88x921(xcfx89) which is the inverse of the noise spatial correlation function matrix KN(xcfx89) with the elements KN(xcfx89; r1, rp). G (xcfx89, rp, r0) is the Green function which describes the propagation channel between the talker with the spatial coordinates r0 and the p-th array microphone. However, experimental data and theoretical analysis show that the coherent signal field model is unrealistic for many limited or confined spaces such as automobile environments where wall irregularities will scatter the signal waves propogating inside the automobile cabin.
A method of reducing noise and a noise reduction apparatus are provided utilizing a microphone array including a plurality of microphone elements for receiving a training signal including a plurality of training signal samples, and a working signal including a plurality of working signal samples. At least one frequency domain convertor is coupled to the plurality of microphone elements for converting the plurality of training signal samples and the plurality of working signal samples to the frequency domain. A signal spatial correlation matrix estimator is coupled to the at least one frequency domain convertor for estimating a signal spatial correlation matrix using the converted plurality of training signal samples, and an inverse noise spatial correlation matrix estimator is coupled to the at least one frequency domain convertor for estimating an inverse noise spatial correlation matrix using the converted plurality of working signal samples. A constrained output generator is coupled to the at least one frequency domain convertor, the signal spatial correlation matrix estimator and the inverse noise spatial correlation matrix estimator for generating a constrained output for the noise reduction apparatus using the converted working signal samples, the estimated signal spatial correlation matrix and the estimated inverse noise spatial correlation matrix.
The noise reduction apparatus may be used in conjunction with or implemented as part of a mobile terminal, a speaker-phone, a speech recognition system, or any other device where noise reduction is desirable.